This invention relates to an antenna system for use in, for example, registration and identification applications.
One example of a typical registration and identification system is an automatic vehicle (AVI) system. The AVI system is used to monitor vehicles for various applications such as for example motorway toll charging, speed monitoring, access to restricted areas of only certain vehicles, crime prevention, etc. The AVI system typically includes a transponder on the vehicle, for example the transponder described in our co-pending application number S/N 08/127,910 (TI-16812); and an antenna system for monitoring the transponder and to register the relevant information relating to the vehicle on which the transponder is mounted. Two typical systems are described in our U.S. Patent No. 5,351,052 (TI-17341) and our co-pending application Ser. No. 08/127,680 (TI-16817). In AVI systems for monitoring motorway traffic there are potentially many vehicles approaching at any one time. If, for example, the system is being used for motorway toll charging it is important that each vehicle is accurately identified and the relevant information stored. For this type of application, it is necessary to have multiple antennas covering the area. Generally, each of the antenna comprise a tuned Loop or LC Circuits. The antennas and feeder cables typically need to be constructed of litze wire and are designed such that the inductivity of the antenna is about 27.mu.H.+-.1.mu.H.
The close proximity of two antennas can cause dead zones in the area to be covered. Forming a multiple antenna of tuned antennas would produce an over critical coupled series of tuned LC networks which could result in detuning of the individual antennas and heavy damping. Obviously this would mean that the system is not capable of registering and identifying all transponders in the field of view of the antennas. The problems caused can, to some extent, be overcome by critical on-location antenna pretuning to ensure that the resonant dead zones are minimized. This can be time consuming, costly and inconvenient.
An auto-tuning system for a tuned antenna system has been used to solve the problem of tuning-on-location and detuning due to metal objects by adding circuitry. This solution is expensive due to the complex circuitry required.
One object of the present invention is to provide an antenna system which overcomes at least some of the disadvantages of known systems.